This article reviews pharmacokinetic studies of IgG administration by intravenous and subcutaneous routes. Intravenous immunoglobulin pharmacokinetics have been studied during replacement therapy for primary and secondary immunodeficiencies and other special circumstances (eg, infection prophylaxis in neonates). Subcutaneous immunoglobulin pharmacokinetics have been studied only during replacement therapy for primary immunodeficiency. Published studies vary greatly with respect to the nature of the patients studied, dose regimens, sampling schedules, and pharmacokinetic models, making comparisons difficult. With either route of administration, there is large variation in individual IgG elimination rates. Periodic measurement of serum IgG concentration is critical to monitor the adequacy of replacement during therapy.